Method for the surface purification of a graphite containing impurities using a dilute aqueous solution of nh4f and h2so4

ABSTRACT

Process for preparing particles of graphite that are purified in surface, from particles of an impure graphite, this process comprises at least one step of treating particles of graphite by means of a diluted aqueous solution of (H 2 SO 4  and NH 4 F), H 2 SO 4  and NH 4 F each being present in the diluted aqueous solution at a weight content representing from 5 to 30% of total weight of the aqueous solution, the quantity of diluted aqueous solution representing from 70 to 95% of the weight of the particles of graphite that undergo purification.

FIELD OF THE INVENTION

The present invention has for object a particularly efficient processfor the purification of the surface of particles of graphite and moreparticularly of the surface of particles of natural graphite. Thisprocess, which allows the elimination of impurities such as Si, Ca, S,Fe that are present in natural graphite, at concentrations that varydepending on the supply source, comprises a step of treating theparticles of graphite by means of a diluted solution of (H₂SO₄ andNH₄F), under economical reaction conditions. This treatment may bepreceded by a step of mechanical treatment. The step of mechanicaltreatment consists in a step of crushing particles of natural graphiteuntil obtaining particles having a predetermined granulometrydistribution. The particles of graphite thus purified in surface can beused for example in the preparation of carbon-lithium based electrodes.

PRIOR ART

The carbon-lithium based negative electrode has recently given rise to alot of interest in the industrial as well as the scientific community.As a matter of fact, the use of such an electrode in a rechargeablebattery allows to solve the crucial problem caused by the metalliclithium electrode which is not easily rechargeable in liquidelectrolytes because of the growth of dendrites when the charge density(C/cm²) and/or the current density (mA/cm²) exceed the limit values thatcondition a good operation of the battery. This major problem hasdelayed the appearance of lithium batteries of standard formats (AA, C,D, etc.) intended for the public at large. The first battery of thistype was marketed in the early 90's by Sony Energytech. This battery isso-called lithium-ion and comprises a negative electrode consisting ofcarbon-lithium.

The principle of operation of this electrode is based on thereversibiliity of lithium insertion between the layers of carbon. Theselayers are characterized by a very high anisotropy of the carbon-carbonbinding forces inside the layers (very highly covalent) and between thelayers (very weak of the Van der Waals type). Because of this, sincelithium is a cation of very small size, it can diffuse rapidly betweenthe 2D layers by forming ionic type bonds with the latter which do notresult in irreversible modifications of the bonds inside the layers.Only a small spreading between the layers is noted, thus allowing toaccommodate the inserted lithium.

It is well known that the reversibility of the electrochemical insertionof lithium in carbon is the more specially favorable that the Li⁺ cationis deprived of its solvation sphere during its transfer from theelectrolytic solution towards the interior of solid carbon. Thus, theco-insertion of DMSO and DME results in a more important separation ofthe planes (>300%) thus contributing to a larger disorganization of thehostess structure. The thus inserted lithium additionally has anapparent weight and volume that is more important, which reduces itsmobility as well as its maximum concentration inside the planes. On theother hand, in a propylene carbonate medium, the ternary compound isvery unstable, the solvent being reduced into gaseous propene which canresult in a violent degradation of the battery.

More recently, it has been shown that carbons with imperfectcrystallinity (turbostratic) could insert lithium in PC or PC-DME mediumwithout co-intercalation of the solvent. The difference inelectrochemical behavior of a highly crystalline graphite and a carbonthat is not well crystallized such as coke treated at a temperaturelower than 1800° C., could be caused by a propene release overvoltagethat is more important in the case of coke. However, a first dischargestep results in the formation of a protector film at the surface of thegranular particles of carbon, which is a product that is obtained bydecomposition of the solvent. Once it is formed, this film has animpedance that is sufficient to prevent the transfer of electron that isnecessary for the progression of reduction of the solvent. But, it ishowever conductor through the Li⁺ ions and because of this it behaves asa solid electrolyte. It is also highly probable that this film takespart in the de-solvation of the Li⁺ ion during its transfer and/or itsreduction at the carbon surface.

The electricity that is consumed during this step cannot be recoveredwhen the current is reversed. The faradic yield of the 1^(st) cycle isconsequently low. The reversible capacity measured during the followingcycles is directly bound to the nature of carbon and to the treatmentthat it has undergone as well as to the nature of the electrolyte.

U.S. Pat. No. A-5,082,818 deals with 1-40 μm graphites. This study isbased on the relation between structure and electrochemistry. However,no information is found that relates to the purity of graphite powder,or still its process of preparation.

U.S. Pat. No. A-5,756,062 discusses the modification of the surface of ahighly purified graphite. The graphite used is however not a crudegraphite directly obtained from a mineral. The chemical modification ofthe graphite is carried out by treatments based on fluorine, chlorine orphosphorus.

The graphite that is conventionally used as electrode material in alithium-ion battery is generally obtained from 2 distinct sources,namely synthetic graphite, or possibly thermally highly purified naturalgraphite, preferably at temperatures higher than 2500° C. Such agraphite, although of excellent quality, is however extremely costly,which has a direct incidence on the final product that is eventuallysold on the market. Moreover, the graphite is only reduced in powderform after having been purified or synthesized, which causes someproblems during the crushing process. Indeed, a homogenous distributionof the particles in the powder is highly altered, since graphite in purestate is very fragile. In fact, one can consider that it has arelatively non-homogenous distribution. If a battery is directlymanufactured with a graphite having such a non-homogenous distribution,it is clear that its life span will be greatly reduced. The alternativeis then to filter the graphite in order to retain only the particleshaving the desired size, which results in additional steps of theprocess, and ultimately, an increase of the cost of the resultingmaterial.

In International Application PCT/CA 01/00233, a process was recentlyproposed allowing purification, for example in surface of graphiteparticles by implementing a process including at least one step ofcrushing graphite until obtaining a size distribution between 1 and 50μm, followed by a step of purifying the particles obtained by chemicalmeans preferably by using an acid treatment preferably with HF or afluorinated derivative allowing the production of HF in the medium,and/or by heat treatment at temperatures typically between 1000 and3000° C. Although it has important advantages with respect to the priorprocesses, this process recommends, as illustrated by the embodimentsreported in PCT Application CA 01/00233, the use of important quantitiesof an acid reactant. These quantities, expressed in weight, are at least3 times higher than that of the graphite particles to be treated. Thisprocess is associated with relatively high exploitation costs andpossibilities of degradation of the internal structure of the graphiteparticles, as often takes place during an exfoliation.

There was therefore a need for providing a process allowing the surfacepurification of graphite particles under economical conditions ofexploitation and without damaging the initial structure of graphite. Ina preferred embodiment, the process should make it possible to obtain agraphite powder with a relatively homogenous size distribution.

SUMMARY OF THE INVENTION

The present invention relates to a process for the preparation ofparticles of graphite that is purified in surface, from graphiteparticles. This process including at least on step of treating theparticles of graphite at a temperature between room temperature and 95 °C., for a period of 5 minutes to 6 hours, by means of an aqueous mixtureof H₂SO₄ and NH₄F, H₂SO₄ and NH₄F each being present in the aqueousmixture in a weight content representing from 5 to 30% of the totalweight of the aqueous mixture of H₂SO₄ and NH₄F, the quantity of aqueousmixture representing from 70 to 95% by weight of the total weight of theparticles of graphite that are subject to purification.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1/5 represents the experimental installation used to carry out thepurification treatment according to an embodiment of the presentinvention.

FIG. 2/5 represents the size distribution carried out with impuregraphite for which the treatment is described in example A hereinafter.

FIG. 3/5 represents the size distribution carried out with purifiedgraphite obtained at the end of the treatment described in example Ahereinafter.

FIG. 4/5 represents the size distribution carried out with the impuregraphite for which the treatment is described in example B hereinafter.

FIG. 5/5 represents the size distribution carried out with purifiedgraphite obtained at the end of the treatment described in example Bhereinafter.

DETAILED DESCRIPTION OF THE INVENTION

A first object of the present invention consists in a process forpreparing particles of graphite that are purified in surface fromparticles of graphite (preferably natural graphite) that containimpurities, whose size is preferably between about 1 and 375 μm, morepreferably still between 1 and 50 μm, said process including at leastone step of treating said particles of graphite at a temperature betweenroom temperature and 95 ° C., preferably at a temperature between 40 and90 ° C., more preferably still at a temperature of about 60 ° C., for aperiod of 5 minutes to 6 hours, preferably for a period of 10 minutes to4 hours, by means of a diluted aqueous solution of (H₂SO₄ and NH₄F),H₂SO₄ and NH₄F each being present in the diluted aqueous solution at aweight content representing from 5 to 30% of the total weight of thediluted aqueous solution, the quantity of diluted aqueous solutionrepresenting from 70 to 95% by weight, preferably from 80 to 90% of theweight of the particles of graphite that are subject to purification,said treatment being preferably carried in a reactor, advantageously inthe presence of a mechanical stirrer preferably of the planetary mixertype, and the diluted aqueous solution of (H₂SO₄ and NH₄F) preferablybeing introduced in the reaction mixture at the same time as theparticles of graphite that are subject to purification.

A second object of the present invention consists in a process forpreparing particles of graphite that are purified in surface fromparticles of graphite (preferably natural graphite) whose size ispreferably between about 1 and 375 μm, more preferably still between 1and 50 μm, said process including:

-   -   a) a step of preparing, preferably at room temperature and under        preferably mechanical stirring, a reaction mixture containing        said particles of graphite (preferably natural graphite), NH₄F,        H₂SO₄ (preferably at 96%) and water, H₂SO₄ and NH₄F each being        present in the diluted aqueous solution of (H₂SO₄ and NH₄F) that        is formed, at a weight content of 5 to 30%, the quantity of        diluted aqueous solution representing from 70 to 95% by weight,        preferably from 80 to 90% by weight of the weight of said        particles of graphite;    -   b) at least one step of treating the mixture prepared in step a)        at a temperature between room temperature and 95° C., preferably        at a temperature between 40 and 90° C., for a period of 5        minutes to 6 hours, preferably for a period of 10 minutes to 4        hours, by means of the diluted aqueous solution of (H₂SO₄ and        NH₄F), said treatment preferably being carried out in the        presence of a mechanical stirring that is preferably of the        planetary mixer type;    -   c) a step of separation, preferably by filtering, of the liquid        phase and the solid phase that are present in the treated        mixture obtained in step b);    -   d) a step of washing the solid phase that is recovered in        step c) by means of a solvent, preferably by means of an aqueous        solvent that is preferably water;    -   e) a step of removing the impurities that are extracted from the        surface of graphite and that are found in the liquid phase        recovered in step c) and/or in the washing solution that is        recovered in step d); and    -   f) an optional step of recycling the purified liquid phase that        is recovered in step e), preferably after enrichment in H₂SO₄        and in NH₄F, during step a) and/or during step b) of said        process.

According to an advantageous embodiment of the processes of theinvention, the particles of graphite subject to purification areobtained by crushing a graphite until obtaining particles of a sizebetween about 1 and 50 μm.

Preferably, the diluted aqueous solution of (H₂SO₄ and NH₄F) that isused comprises from 5 to 30% of H₂SO₄ and from 5 to 30% of NH₄F, it isadded preferably at the start of the purification treatment, undermechanical stirring, the residual acid mixture is neutralized preferablyin situ, preferably with NaOH or by complete dilution with H₂O, and thetemperature is preferably kept constant when carrying out the process.

According to an advantageous embodiment, the crystallographic structureof graphite is unchanged before and after leaching as shown for exampleby a control of the crystallographic parameters L_(c) and L_(a) ofgraphite carried out respectively by X ray diffraction and by Ramanspectroscopy.

The types of graphite that are advantageously used, alone or inadmixture, within the scope of the processes of the invention are:

-   -   graphite of the type StratminGraphite (Canada), rich in calcium;    -   graphite of the Chinese type, rich in silicon;    -   graphite of the type Lake Knife (Canada), rich in sulfur; and    -   graphite of the Brazilian type, rich in iron impurities and all        worldwide source.

According to an advantageous embodiment, steps (a) and (b) are carriedout under conditions suitable to give particles of graphite having oneof more of the following additional properties:

-   -   an interplanar distance d₀₀₂ measured by X-ray, that varies        between 3.35 Å and 3.38 Å;    -   a specific surface that varies between 0.4 and 55 m²/g; and    -   a coefficient of purity measured by backscattering with the        secondary electrons that varies between 98.5% and 99.99%.

Purification is preferably carried out in a bath preferably an aqueousbath.

According to an advantageous variant, the particles of purified graphiteobtained during said process are, in an additional step, conditioned inthe form of a carbon anode for an electrochemical generator comprisingan alkaline or alkaline-earth metal. Preferably, the metal is lithium.

The anode may advantageously be prepared by mixing the particles ofgraphite with a binder and with a solvent, and by spreading the mixtureobtained on a metallic collector.

Purification, on the other hand, is carried out so as to remove saidimpurities and the corrosion sites in surface only.

Preferably, an oxidizing acid that is preferably of the type NHO₃ isadded in the reaction mixture to obtain an exfoliated and purified (insurface) graphite in a single step.

According to this process, H₂SO₄ is present in the diluted aqueoussolution of (H₂SO₄ and NH₄F) at the rate of at least 80% by weight ofthe total weight of (H₂SO₄ and NH₄F) and NH₄F is present in the dilutedaqueous solution of (H₂SO₄ and NH₄F) at the rate of at most 20% byweight of the total weight of (H₂SO₄ and NH₄F).

Still more advantageously, the temperature/time gradient, duringtreatment, is between 10 and 90 degrees Celsius per hour.

A third object of the present invention consists of particles of nonpurified natural graphite of a size between about 1 and 50μm and whosesurfaces have no impurity nor corrosion site, said particles beingobtained by one of the processes according to the present invention.

A fourth object of the present invention consists of carbon anodes basedon particles of graphite that can be obtained by one of the processesaccording to the invention.

The preferred anodes are those obtained by means of particles of crushedand purified natural graphite comprising at least one of the followingadditional properties:

-   -   an interplanar distance d₀₀₂ obtained by X-ray that varies        between 3.35 Å and 3.38 Å;    -   a specific surface that varies between 0.4 and 55 m ²/g; and    -   a coefficient of surface purity measured by backscattering with        the secondary electrons, that varies between 98.5% and 99.99%.

A fifth object of the present invention consists of electrochemicalbatteries comprising an anode according to the present invention.

Among the batteries according to the invention that are particularlyperforming, those of the lithium-ion type may be mentioned.

The present invention that relates to a new process for purifying thesurface of graphite particles constitutes an improvement of the processgenerally described in PCT Application CA 01/00233. The content ofPCT/CA 01/00233 Application is incorporated in the present applicationby reference.

While PCT/CA 01/00233 Application illustrates the use of the method ofpurifying graphite under strong energetic and reactive conditions,namely a treatment temperature of the order of 90 to 250° C. and aduration of acid treatment of at least 180 minutes, it has surprisinglybeen found that a selection of certain operating parameters that differfrom those previously identified as preferred allows to obtain betterpurification yields, and moreover, under more interesting economicalconditions.

A new method was thus drawn up to produce purified graphite in the formof small particles that can be used in an electrochemical battery, forexample of the lithium-ion type, while having a relatively homogenoussize distribution. Such a graphite may also be used in other types ofapplications as electronic conductor in a cathode (batteries) or incombustion batteries in the nuclear field.

The present invention concerns a method for the chemical purification ofthe impurities that are found at the surface of an impure graphite suchas natural graphite, exactly where the passivation film will be formed.The present method allows to remove the impurities that are capable ofbeing prejudicial to the formation of the passivation film and to thecycling of the carbon-lithium anode. The crushing process is carried outbefore purification since it allows to obtain a better control of thesize and distribution of the particles, hence a more homogeneous powderrequiring no filtering to remove particles that are too large or toosmall.

The subsequent purification step aims essentially at removing from thesurface of the particles of graphite, the impurities that areresponsible for electronic conductivity, such as the compoundscomprising silicon and iron. These compounds are also responsible fordoping and reduction through lithium of the compounds that contain same.These phenomena should absolutely not be present, or at least theyshould be highly minimized, in the passivation film that will be formedat the surface of the electrode, since the latter will cause adegradation of the efficiency of the battery, and ultimately ashort-circuit. On the other hand, the presence at the surface ofimpurities promoting ionic conductivity, such as calcium fluoride, hasno negative influence on the performances of the graphite electrodebecause of its strong ionic character which has little effect on aconductivity of electronic nature.

The impurities that are present in a graphite mineral are generally thefollowing (in a decreasing order): Si >Ca >Fe >S >Al. As previouslymentioned, the compounds comprising silicon must be removed since on theone hand, lithium reduces or dopes the compounds comprising silicon (forexample SiO₂, SiO and Si metal), and on the other hand, the siliconcompounds are electronic conductors. This latter property is totallyincompatible with the properties of the passivation film, whichrepresents a key-element of a good carbon-lithium anode that ischaracterized by a long life span.

A first embodiment of the invention consists in a process for preparinggraphite particles that are purified in surface, from graphite particles(preferably natural graphite) including impurities, of a size that ispreferably between 1 and 375 μm, more preferably still between 1 and 50μm. This process includes at least one step of treating said particlesof graphite at a temperature between room temperature and 95° C.,preferably at a temperature between 40 and 90° C., more preferably stillat about 60° C., for a period of 5 minutes to 6 hours, preferably for aperiod of 10 minutes to 4 hours, by means of a diluted aqueous solutionof (H₂SO₄ and NH₄F), H₂SO₄ and NH₄ each being present in the dilutedaqueous solution at weight content representing from 5 to 30% of thetotal weight of the diluted aqueous solution, the quantity of dilutedaqueous solution representing from 70 to 95% by weight, preferably 80 to90% by weight of the particles of graphite subject to purification, saidtreatment being preferably carried out in a reactor, advantageously inthe presence of a mechanical stirring preferably of the planetary mixertype, and the diluted aqueous solution of (H₂SO₄ and NH₄F) is preferablyintroduced into the reaction mixture at the same time as the particlesof graphite that are subject to purification.

A second preferred embodiment of the present invention consists in aprocess for preparing particles of graphite that are purified in surfacefrom graphite particles (preferably natural graphite) includingimpurities, of a size preferably being between 1 and 375 μm, morepreferably still between 1 and 50 μm, said process including:

-   -   a) a step of preparing, preferably at room temperature and under        mechanical stirring, a reaction mixture containing said        particles of graphite (preferably natural graphite), NH₄F, H₂SO₄        (preferably at 96 %) and water, H₂SO₄ and NH₄F each being        present in the diluted aqueous solution of (H₂SO₄ and NH₄F) that        is formed, at a weight content of 5 to 30%, the quantity of        diluted aqueous solution representing from 70 to 95% by weight,        preferably from 80 to 90% by weight of said particles of        graphite;    -   b) at least one step of treating the mixture prepared in step a)        at a temperature between room temperature and 95° C., preferably        at a temperature between 40 and 90° C., more preferably still at        a temperature of about 60° C., for a period of 5 minutes to 6        hours, preferably for a period of 10 minutes to 4 hours, by        means of the diluted aqueous solution of (H₂SO₄ and NH₄F) that        is formed, said treatment being preferably carried out in the        presence of a mechanical stirring that is preferably of the        planetary mixer type;    -   c) a step of separating, preferably by filtering, the liquid        phase and the solid phase that are present in the treated        mixture obtained in step b);    -   d) a step of washing the solid phase that is recovered in        step c) by means of a solvent, preferably by means of an aqueous        solvent that is preferably water;    -   e) a step of removing impurities that are extracted from the        surface of the graphite and that are present in the liquid phase        that is recovered in step c) and/or in the washing solution that        is recovered in step d);    -   f) a step of recycling the purified liquid phase that is        recovered in step e), preferably after enrichment in H₂SO₄ and        in NH₄F, in step a) and/or in step b) of said process.

Preferably, H₂SO₄ is present in the diluted aqueous solution of (H2SO₄and NH₄) at the rate of at least 80% by weight of the total weight of(H₂SO₄ and NH₄) and NH₄F is present in the aqueous solution at the rateof at most 20% by weight of the total weight of (H₂SO₄ and NH₄).

The present method of purification does not modify the size of thegranules as determined by the crushing process. There is therefore noagglomeration of the particles. These particles are therefore free andthus form a homogeneous mixture with the binder in order to obtain anelectrode of good quality (roughness, thickness, porosity, etc).

The step of crushing may be carried out according to any technique knownto one skilled in the art. Such techniques include for example jetmilling, air milling and ball milling.

The following examples are given to show the performances and advantagesof the process of the present invention, for example with respect to theprocess described in PCT Application CA 01/00233. They should in no waybe interpreted as bringing any kind of limitation to the scope of theobject of the present invention.

EXAMPLE ACCORDING TO THE PROCESS DESCRIBED IN PCT/CA01/00233 Example 1

30 grams of a natural graphite StratminGraphite (Lac des îles—Québec)having an initial particle size of 375 μm are crushed by an“air-milling” process until the particles reach a size of 10 μm. Theaverage size obtained for the particles (D50%) is 10.52 μm. The Gaussiandistribution of graphite has a single maximum without any shoulder. Thesize distribution was determined by means of the Microtrac™ particleanalyzer manufactured and sold by Leeds & Northrul. Methanol was used ascarrier liquid. Subsequently, the crushed graphite was leached in areactor filled with 106.5 ml of an aqueous bath of HF 30%. Thetemperature of the mixture was fixed to 90° C., and the leaching timewas 180 minutes. The graphite was thereafter filtered, washed with fullwater, and the powder was dried during 24 hours at 120° C.

The graphite powder obtained is analyzed by backscattering coupled withEDX. No exfoliation of the particles was observed. On the other hand,EDX analysis shows that most of the remaining impurities consist ofcalcium. Purity of this sample is 99.6%, as obtained by the method ofanalysis of the remaining ashes from the impurities.

This graphite is mixed with the fluorinated polyvinylidene binder (PVDS)(Kruha: KF-1700) and n-methyl pyrolidone in a weight ratio of 90:10.This mixture is applied on a copper collector by the Doctor Blade™method.

The graphite electrode thus obtained is dried under vacuum at 120° C.during 24 hours and is mounted in a button cell of the 2035 type. ACelgard™ 2300 separator soaked with the electrolyte 1M LiPF₆+EC/DMC:50/50 (ethylene carbonate+dimethyl carbonate) is used. Metallic lithiumis used as reference and as counter-electrode.

Electrochemical tests were carried out at room temperature. Curves ofdischarge—charge were obtained between 0 and 2.5 volts in C/24. Thefirst coulomb yield is 85%, which is higher than commercial graphiteused in lithium-ion batteries (typically 81%). The reversible capacityis 365 mAH/g which is equivalent to x=0.98 in Li_(x)C₆. This obtainedvalue is very close to the theoretical value for graphite (372 mAh/g).No negative effect associated with the presence of the residualimpurities of Ca is noted.

Example 2

30 grams of a natural graphite StratminGraphite (Lac des îles—Québec)having an initial particle size of 375 μm are crushed by an“air-milling” process until the particles reach a size of 10 μm. Thegraphite is then leached in a bath consisting of 106.5 ml of an aqueousmixture comprising 30% H₂SO₄ and 30% HF. Then, 106.5 ml of the acidmixture is heated to 90° C. and 30 g of graphite are then added to thesolution. The graphite is leached during 180 minutes in the reactor. Thesolid is thereafter filtered, washed in full water, and dried at 120° C.during 24 hours. The size (D50%) of the particles obtained is 10.92 mm,and this before and after purification. The gaussian distribution ofgraphite has a single maximum, without any shoulder.

An analysis of the impurities of this graphite by EDX shows a majorpresence of the elements Ca and F. An analysis of the residual ashesfrom the impurities present in and at the surface of graphite shows apurity of 99.68%. Preparation of the electrode and electrochemical testsare carried out according to the procedure described in example 1.

The coulomb efficiency of the first cycle is 90%. The irreversibleplateau of the passivation film is normally formed at about 800 mV. Thismeans that the elements Ca, F or CaF₂ did not influence the formation ofthe passivation film. The reversible capacity of the graphite is 356mAh/g, which is equivalent to x=0.96 in Li_(x)C₆. formation.

Example 3

30 grams of a natural graphie StratminGraphite (Lac des îles—Québec) aretreated similarly as in example 2 except for the acid concentration ofHF which, in the aqueous bath, is now 20%. An analysis of the impuritiesof this graphite by EDX shows the major presence of the elements Ca andF. An analysis of the residual ashes from the impurities that arepresent in and at the surface of the graphite shows a purity of 99.75%.The preparation of the electrode and the electrochemical tests areidentical to the procedures described in example 1.

Coulomb efficiency of the first cycle is 89%. The irreversible plateauof the passivation film is normally formed at about 800 mV. Thereversible capacity of the graphite is 365 mAh/g and the equivalent ofx=0.98 according to the formation of Li_(x)C₆.

Example 4

30 grams of a natural graphite StratminGraphite (Lac des îles—Québec)are treated similarly as in example 2, except for the acid concentrationin the aqueous bath, that is now 10% HF. Preparation of the electrodeand electrochemical tests are identical to the procedures described inexample 1.

The coulomb efficiency of the first cycle is 75%. The irreversiblecapacity of 106.7 mAh/g is very high compared to that of the graphite ofexample 2 and 3, respectively leached with HF 30% and HF 20%. Thereversible capacity is 318 mAh/g and is equivalent to x=0.85 in theformation of Li_(x)C₆.

Example 5

30 grams of a natural graphite StratminGraphite (Lac des îles—Québec)are treated similarly as in example 2, except for the mixture H₂SO₄-HFin which HF is replaced by NH₄F. The aqueous solution of H₂SO₄ and NH₄F,therefore includes 30% H₂SO₄ and 30% NH₄F.

The aqueous mixture of (H₂SO₄ and NH₄F) content represents, as in thepreceding examples, about 3 times the weight of the treated granules ofgraphite.

Treatment with the acid solution is carried out at 90° C. during 3hours.

An analysis of the impurities of this graphite by EDX shows the majorpresence of the elements Ca and F. An analysis of the residual ashes ofthe impurities on this graphite shows a purity of 99.64%. Preparation ofthe electrode and electrochemical tests are identical to the proceduresdescribed in example 1.

The coulomb efficiency of the first cycle is 90%. The irreversiblecapacity of graphite is 44 mAh/g. The reversible capacity is 352 mAh/gand is equivalent to x=0.96 in the formation of Li_(x)C₆.

Example 6

30 grams of a natural graphite StratminGraphite (Lac des îles—Québec)consisting of particles having a size of 10 μm are leached in twostages. First, with an aqueous solution of 30% HCl, then with an aqueoussolution of 30% HF. For each leaching, 106.5 ml of the acid solution areheated at 90° C., and 30 g of graphite are then added. The graphite isleached during 180 minutes in the reactor. The solid is filtered, washedwith full water, and dried at 120° C. during 24 hours.

The size (D50%) is 10.02 μm. The gaussian distribution of the graphitehas a single maximum without any shoulder.

An analysis of the impurities of this graphite by EDX shows a totalabsence of the elements Si and Ca. The main element obtained as impurityis sulfur. An analysis of the residual ashes from the impurities ongraphite shows a purity of 99.99%. Preparation of the electrode andelectrochemical tests are identical to the procedures described inexample 1.

The coulomb efficiency of the first cycle is 88%. The irreversibleplateau of the passivation film is normally formed at about 800 mVolts.It may therefore be concluded that the presence of sulfur has no badeffects when the passivation film is formed. The reversible capacity ofgraphite is 357 mAh/g and is equivalent to 3=0.96 in the formulation ofLi_(x)C₆.

EXAMPLES OF USING THE PURIFICATION PROCESS ACCORDING TO THE INVENTION

For each test, 150 g of a graphite that originates from Brazil, i.e.rich in impurities (in quantity of decreasing importance) of the typeAl, Si and Fe, are placed in a reactor. The average size of theparticles of graphite (D50) is 20 μm. Leaching is carried out for 4hours. A sample of 50 g is taken after 1, 2 and 4 hours, to checkkinetic evolution. An analysis of the ashes is then completed on eachsample. The results of these analyses are given in the followingparagraphs.

First, an aqueous solution containing 1.5% by weight of NH₄F and 15% byweight of H₂SO₄; was tested at different temperatures. The percentage ofaqueous solution of NH₄F and of H₂SO₄ used represents 20% by weight ofthe weight of graphite treated. The results are presented in thefollowing table. Each sample that is analyzed is identified by a testnumber. % ashes NH₄F H₂SO₄ T Time Test (%) (%) (° C.) 0 h 1 h 2 h 4 h1-3 1.5 15 20 1.88 0.31 0.25 0.22 4-6 1.5 15 60 1.88 0.08 0.05 0.03 7-91.5 15 90 1.88 0.01 0.01 0.01

At 90° C., the reaction is very fast, even at low concentrations ofNH4F. It can be seen that the reaction is nearly complete at 60° C. Byincreasing the NH4F concentrations, the following results have beenobtained at 60 and 20° C. It is surprisingly noted that the intendedlevel for the surface purification of the particles of graphite, i.e. anamount of impurities lower than or equal to 0.03%, is reached byproceeding at a temperature lower than 90° C. and preferably of theorder of 60° C. % ashes NH₄F H₂SO₄ T Time Test (%) (%) (° C.) 0 h 1 h 2h 4 h 10-12 2.5 15 60 1.88 0.09 0.06 0.02 13-15 15 15 20 1.88 0.09 0.050.03 16-18 5 5 60 1.88 0.08 0.06 0.05

By comparing tests 12 and 18, both carried out at 60° C., it appearsthat a decrease of the acid concentration has an important impact.Indeed, residual ash percent is higher with more NH₄F and less H₂SO₄.Test 15 at 20° C., shows that, even with a very strong concentration ofthe two reactants, the reaction is still not completed after 4 hours.

It therefore appears surprisingly that particularly interestingperformances are obtained by carrying out the purification treatmentwith small quantities of an aqueous solution that is weakly acid and ata temperature between room temperature and about 60° C.

It also surprisingly appears that the time of treatment may besubstantially decreased when operating at temperatures near 90° C.

EXAMPLES OF USE OF THE PROCESS ACCORDING TO THE INVENTION WITH PARTICLESOF GRAPHITE OF SPHERICAL SHAPE Example A

The experimental installation used is represented in FIG. 1. Itcomprises for example a polypropylene reactor, a graphite exchanger anda pump made of teflon. In view of the presence of the toxic gas HF,special safe pieces of equipment are used, such as a full-face mask withcartridge for HF, an apron, boots and rubber gloves.

85.6 liters of water are first placed in the reservoir, and 16.15 kg ofH₂SO₄ (96%) are thereafter added. If the acid that is available is notat 96%, the volume of water is adjusted accordingly to obtain a finalsolution at 15% H₂SO₄. For example, for 15.8 kg of a 98% acid, 86 litersof water are added and for 15.8 kg of a 93% acid 85.1 liters of waterare added. The water addition should be carried out slowly to preventthe reaction from being too violent.

A quantity of 1.55 kg of NH₄F is then added into sulfuric acid. Thesolution is thereafter heated at 90° C. with the graphite exchanger.Then, 20 kilograms of a spherical Chinese graphite obtained byMechano-fusion (Hozokawa) whose size is 12 μm, are added in the solutionand the mixture obtained is stirred.

The operating mixture is kept at 90° C. during 1 hour, and the solidportion is filtered and washed with a large amount of water. Thesolution that is recovered is then neutralized with lime.

As shown by the experimental results reported in FIGS. 2 and 3 annexedhereto, the total purity of the particles of graphite which was 96.9%before treatment, reaches a value of 99.4% after treatment according tothe process of the invention.

Example B

The same installation and the same safety pieces of equipment as inexample A were used. The same operation sequence is repeated with asecond sample of 20 kg and with particles of graphite whose size is 20μm obtained by Mechano-fusion (Hozokawa).

As shown by the experimental results reported in FIGS. 4 and 5 annexedhereto, the total purity of the particles of graphite which was 97%before treatment, reaches a value of 99.1% after treatment according tothe process of the invention.

Although the present invention was described by means of specificoperating steps, it is understood that many variations and modificationsmay be associated with these steps, and the present application aims atcovering such modifications, uses and adaptations of the presentinvention, following in general the principles of the invention andincluding any variation of the present description which will becomeknown or conventional in the field of activity in which this applicationis found, and that may be applied to the above mentioned essentialelements, in accordance with the following claims.

1. Process for preparing particles of graphite that are purified insurface, from particles of graphite that contain impurities, saidprocess including at least one step of treating said particles ofgraphite, by means of a diluted aqueous solution of (H₂SO₄ and NH₄F),H₂SO₄ and NH₄F each being present in the diluted aqueous solution in anamount by weight representing 5 to 30% of the total weight of thediluted aqueous solution, the quantity of diluted aqueous solution usedfor the treatment representing 70 to 95% by weight of the total weightof the particles of graphite that are subjected to purification. 2.Process according to claim 1, in which the particles of graphite to bepurified are particles of a natural graphite.
 3. Process according toclaim 1, in which the particles of graphite have a size between 1 and375 μm.
 4. Process according to claim 3, in which the size of theparticles of graphite to be purified is between 1 and 50 μm.
 5. Processaccording to claim 1, in which the step of treating particles ofgraphite is carried out at a temperature between room temperature and95° C.
 6. Process according to claim 5, in which the treatment iscarried at a temperature between 40 and 90° C.
 7. Process according toclaim 6, in which the treatment of the particles of graphite is carriedout at a temperature of about 60° C.
 8. Process according to claim 1, inwhich the treatment of the particles of graphite is carried out for aperiod of 5 minutes to 6 hours.
 9. Process according to claim 8, inwhich the treatment of the particles of graphite is carried out for aperiod of 10 minutes to 4 hours.
 10. Process according to claim 1, inwhich the quantity of diluted aqueous solution in the treatment steprepresents from 80 to 90% of the total weight of the particles ofgraphite that are subjected to purification.
 11. Process according toclaim 1, carried out in a reactor, in the presence of mechanicalstirring.
 12. Process according to claim 11, in which the mechanicalstirring is of the planetary mixer type.
 13. Process according to claim1, in which the diluted aqueous solution of (H₂SO₄ and NH₄F) isintroduced into the reaction mixture at the same time as the particlesof graphite that are subjected to purification.
 14. Process forpreparing particles of graphite that are purified in surface fromparticles of impure graphite, said process including: a) a step ofpreparing a reaction mixture containing said particles of impuregraphite, NH₄F, H₂SO₄ and water, H₂SO₄ and NH₄F each being present inthe diluted aqueous solution of (H₂SO₄ and NH₄F) that is formed, at aweight content of 5 to 30%, the quantity of diluted aqueous solutionrepresenting from 70 to 95% by weight of the weight of said particles ofgraphite; b) at least one step of treating the mixture prepared in stepa) at a temperature between room temperature and 95° C., for a period of5 minutes to 6 hours, by means of the diluted aqueous solution of (H₂SO₄and NH₄F); c) a step of separation of the liquid phase and the solidphase that are present in the treated mixture obtained in step b); d) astep of washing the solid phase that is recovered in step c), by meansof a solvent; e) a step of removing the impurities that are extractedfrom the surface of graphite and that are found in the liquid phaserecovered in step c) and/or in the impurities that are present in thewashing solution that is recovered in step d).
 15. Process according toclaim 14 including an additional step of recycling the purified liquidphase that is recovered in step e), in step a) and/or in step b) of saidprocess.
 16. Process according to claim 14, for the preparation ofparticles of natural graphite that are purified in surface.
 17. Processaccording to claim 14, in which the particles of graphite to be purifiedhave a size between about 1 and 375 μm.
 18. Process according to claim17, in which the particles of graphite to be purified have a sizebetween 1 and 50 μm.
 19. Process according to claim 14, in which step a)is carried out at room temperature and under mechanical stirring. 20.Process according to claim 14, in which the quantity of diluted aqueoussolution (H₂SO₄ and NH₄F) that is added during step a) represents 80 to90% of the total weight of said particles of graphite.
 21. Processaccording to claim 14, in which the treatment of the mixture preparedduring step a) is carried out at a temperature between room temperatureand 95° C.
 22. Process according to claim 14, in which step a) and/ orstep b) last(s) from 5 minutes to 6 hours.
 23. Process according toclaim 22, in which the duration is from 10 minutes to 4 hours. 24.Process according to claim 14, in which step b) is carried out in thepresence of mechanical stirring.
 25. Process according to claim 14, inwhich the solvent, used in step d) for washing the solid phase, iswater.
 26. Process according to claim 14, in which the purified liquidphase that is recovered in step a) is enriched in H₂SO₄ and in NH₄F, instep a) and/or in step b) of said process, before recycling.
 27. Processaccording to claim 1, in which the particles of graphite that aresubject to purification were obtained by crushing a graphite untilobtaining particles of a size between about 1 and 50 μm.
 28. Processaccording to claim 1, comprising at least one of the followingcharacteristics: the diluted aqueous solution of (H₂SO₄ and NH₄F) used,comprises from 5 to 30% of H₂SO₄ and from 5 to 30% of NH₄F; the dilutedaqueous solution of (H₂SO₄ and NH₄F) used, is added at the start of thepurification treatment, under mechanical stirring; the residual acidmixture is neutralized preferably in situ with NaOH or by completedilution with H₂O; and the temperature is kept constant during theentire duration of the process.
 29. Process according to claim 1,wherein the crystallographic structure of the graphite is unchangedbefore and after leaching as shown by a control of the crystallographicparameters L_(c) and L_(z) of the graphite carried out respectively by Xray diffraction and/or by Raman spectroscopy.
 30. Process according toclaim 1, wherein the graphite is selected from the group consisting of aStratminGraphite (Canada) type rich in calcium, graphites of the Chinesetype rich in silicon, a graphite of Lake Knife (Canada) rich in sulfur,graphites of the Brazilian type rich in impurities of the iron type andany available graphite from a natural source.
 31. Process according toclaim 2, wherein steps (a) and (b) are carried out under conditionssuitable to give particles of graphite having at least one of thefollowing additional properties: an interplanar distance d₀₀₂ obtainedby X-ray that varies between 3.35 Å and 3.38 Å, all included; a specificsurface that varies between 0.4 and 55 m²/g, all included; and acoefficient of surface purity measured backscattering with the secondaryelectrons that varies between 98.5% and 99.99%, all included. 32.Process according to claim 1, wherein the purification is carried out ina bath, preferably in an aqueous bath.
 33. Process according to claim 1,wherein the purified particles of graphite obtained in the course ofsaid process are, in a supplementary step, conditioned in the form of acarbon anode for rechargeable electrochemical generator comprising analkaline or alkaline-earth metal.
 34. Process according to claim 33,characterized in that the metal is lithium.
 35. Process according toclaim 33 wherein the anode is prepared by mixing the particles ofgraphite with a binder and with a solvent, and in that the mixtureobtained is spread on a metallic collector.
 36. Process according toclaim 1, wherein the purification is carried out so as to remove saidimpurities and the corrosion sites, in surface only.
 37. Particles ofnon purified natural graphite of a size between about 1 and 50 μm andwhose surfaces contain no impurity and no corrosion site, said particlesbeing capable of being obtained by one of the processes according toclaim
 1. 38. Carbon anode based on particles of graphite according toclaim
 37. 39. Anode according to claim 38, characterized in that thecrushed and purified particles of graphite comprise at least one of thefollowing additional properties: an interplanar distance d₀₀₂ obtainedby X-ray that varies between 3.35 Å and 3.38 Å, all included; a specificsurface that varies between 0.4 and 55 m²/g, all included; and acoefficient of surface purity measured by backscattering with thesecondary electrons that varies between 98.5% and 99.99%, all included.40. Electrochemical battery comprising an anode according to claim 38.41. Battery according to claim 40, characterized in that it is of thelithium-ion type.
 42. Process according to claim 1, in which anoxidizing acid is added in the reaction mixture to obtain, in a singlestep, exfoliated and purified graphite (in surface).
 43. Processaccording to claim 42, in which the oxidizing acid if of the HNO₃ type.44. Process according to claim 1, in which H₂SO₄ is present in thediluted aqueous solution of (H₂SO₄ and NH₄F) at the rate of at least 80%by weight of the total weight of (H₂SO₄ and NH₄F) and NH₄F is present inthe diluted aqueous solution of (H₂SO₄ and NH₄F) at the rate of at most20% by weight of the total weight of (H₂SO₄ and NH₄F).
 45. Processaccording to claim 1, in which during treatment the temperature/timegradient is between 10 and 90 degrees Celsius per hour.
 46. Processaccording to claim 14, in which step c) is a step of filtering of theliquid phase and the solid phase that are present in the treated mixtureobtained in step b).
 47. Process according to claim 28, in which theresidual acid mixture is neutralized in situ with NaOH or by completedilution with H₂O.
 48. Process according to claim 32, characterized inthat the purification is carried out in an aqueous bath.